Retroreflective structures have been incorporated into a wide variety of different articles in order to make such articles more visible under low light conditions. A typical retroreflective structure works by reflecting a substantial portion of incident light back towards the illumination source. As a result, the underlying article or imagery printed on the retroreflective structure is easily seen by an observer in or near the path of the reflected light. For example, one common application for retroreflective articles involves covering the surface of a traffic sign with a layer of retroreflective sheeting. At night, incident light from the headlights of an oncoming motor vehicle bathes the sign in light, which is then reflected back to the driver of the motor vehicle. Information on the sign is easily read as a result.
Of course, traffic signs are not the only application for retroreflective products. Other examples of articles that benefit from retroreflective characteristics include lens elements on pavement markers (particularly raised pavement markings), street name signs, pavement marking tapes, reflectors on bicycles, conspicuity markings for motor vehicles and trains, traffic cones, license plates, self adhesive stickers (such as the validation stickers that are affixed to license plates or windshields), commercial advertising signs, clothing, barrels, barricades, and the like.
On many retroreflective articles, the exposed front surface is formed from thermoplastic or thermosetting polymers. Examples of such polymers include thermosetting or thermoplastic polycarbonate, poly(meth)acrylate (PMMA), ethylene/acrylic acid copolymers (EAA), polyvinyl chloride (PVC), polyurethane, polyester, polyamide, polyimide, phenoxy, phenolic resin, cellulosic resin, polystyrene, styrene copolymer, epoxy, and the like. Many of these thermoplastic and thermosetting polymers have excellent rigidity or flexibility (depending upon the desired application), dimensional stability, and impact resistance, but unfortunately have poor abrasion resistance. Consequently, retroreflective articles incorporating these polymers are susceptible to scratches, abrasion, and similar damage.
To protect the surfaces of retroreflective articles from physical damage, a tough, abrasion resistant "hardcoat" layer may be coated onto one or more portions of the retroreflective surfaces of such articles. Many previously known hardcoat layers incorporate a binder matrix formed from radiation curable prepolymers such as (meth)acrylate functional monomers. Such hardcoat compositions have been described, for example, in Japanese patent publication JP02-260145, U.S. Pat. No. 5,541,049, and U.S. Pat. No. 5,176,943. One particularly excellent hardcoat composition is described in WO 96/36669 A1. This publication describes a hardcoat formed from a "ceramer" used, in one application, to protect the surfaces of retroreflective sheeting from abrasion. As defined in this publication, a ceramer is a hybrid, polymerizable composite (preferably transparent) having inorganic oxide particles, e.g., silica, of nanometer dimensions dispersed in an organic binder matrix.
Many ceramers are derived from aqueous sols of inorganic colloids according to a process in which a radiation curable binder matrix precursor (e.g., one or more different radiation curable monomers, oligomers, or polymers) and other optional ingredients (such as surface treatment agents that interact with the colloids of the sol, surfactants, antistatic agents, leveling agents, initiators, stabilizers, sensitizers, antioxidants, crosslinking agents, and crosslinking catalysts) are blended into the aqueous sol. The resultant composition is then dried to remove substantially all of the water. The drying step sometimes is referred to as "stripping". An organic solvent may then be added, if desired, in amounts effective to provide the composition with viscosity characteristics suitable for coating the composition onto the desired substrate. After coating, the composition can be dried to remove the solvent and then exposed to a suitable source of energy to cure the radiation curable binder matrix precursor.
Light transmission to and from a retroreflective article, such as a traffic sign or a raised pavement marker, can be impaired by water droplets. Dew formation can be particularly problematic, because dew condenses onto signs predominantly at nighttime when the illuminating characteristics of retroreflective sheetings are most beneficial. Water droplets on traffic signs can significantly alter the path of incident and retroreflected light. This can make information on the sign much more difficult for passing motorists to read. Thus, the elimination or reduction of small beaded water droplets on the surface of a sign increases retroreflectance and readability by reducing the extent to which incident light is scattered or otherwise misdirected by water droplets on the surface of a sign.
To hamper water droplet formation in moist conditions, coatings have been applied to signs to evenly spread the water over the coating. Water-spreading coatings typically include inorganic particles and may also include an organic binder. For example, U.S. Pat. No. 4,576,864 to Krautter et al. discloses a water-spreading layer that is composed of colloidal particles of a metal or silicon oxide in which the water-spreading layer is adhered to a plastic substrate by an adhesive. U.S. Pat. Nos. 4,478,909 to Taniguchi et al. and 5,134,021 to Hosono et al. disclose an anti-fogging coating having finely divided silica particles dispersed in a matrix of polyvinyl alcohol and an organosilicon alkoxy compound or hydrolysates thereof. U.S. Pat. No. 4,409,285 to Swerdlow discloses a water-spreading coating containing a mixture of large and small inorganic particles containing colloidal silica or alumina. U.S. Pat. No. 4,481,254 to Fukishima et al. discloses an agricultural plastic film containing an olefin resin and an amorphous hydrated aluminum silicate gel. U.K. Patent Application GB 2,249,041A to the Imperial College of Science, Technology and Medicine, discloses a modified hydrophobic plastic surface that has been subjected to an oxidation treatment and has a surface layer of colloidal hydrous metal oxide. Japanese Patent Kokai Publication No. HEI-3-50288 to Yamagishi et al. discloses an anti-fogging composition containing a mixture of positively charged colloidal silica and alumina particles with a water-soluble aluminum salt and a nonionic surfactant. U.S. Pat. Nos. 5,073,404, 4,844,946 and 4,755,425 to Huang disclose a retroreflective sheeting that has a transparent coating containing colloidal silica and a polymer selected from aliphatic polyurethanes, polyvinyl chloride copolymers and acrylic polymers.
Other water-spreading layers are known that do not require inorganic particles. For example, U.S. Pat. No. 5,244,935 to Oshibe et al. discloses an ultraviolet curable anti-fogging composition agent containing an acrylate or acrylamide block copolymer having a hydrophilic polymer segment and a hydrophobic polymer segment, a photopolymerizable compound, and a photoinitiator. The photopolymerizable compound has the formula CH.sub.2.dbd.CHCOO(CH.sub.2 CRHO).sub.n OCCR.dbd.CH.sub.2 ; when n=0, anti-fogging properties were not exhibited and when n&gt;30, the resulting film was weak. U.S. Pat. No. 5,316,825 to Nakai et al. discloses an anti-fogging film made of a transparent synthetic resin having micro concavities of at most 10 .mu.m in depth and 20 .mu.m in width.
Other workers have reported that anti-fogging properties can be imparted to glass or surface-activated plastic substrates by reacting the substrate surfaces with silanol or siloxane-functionalized polymers or fluoropolymers. European Patent Application No. 0 620 255 A1 to Luckey, Ltd. reports that anti-fogging coatings can be produced from mixtures of an epoxy functionalized organosiloxane, an amino functionalized organosiloxane, a hydrophilic methacrylate monomer, and a curing catalyst. U.S. Pat. No. 5,270,080 to Mino et al. discloses anti-fogging compositions composed of silanol-functionalized fluoropolymers. European Patent Application Nos. 0 491 251 A1 and 0 492 545 A2 to Matsushita Electric Industrial Co. report water-repelling, oil-repelling anti-fogging films that are made from siloxy-functionalized hydrophobic compounds. These references report that plastic surfaces can be made reactive to hydroxyl groups or hydrophilic by corona treating the surface.
Other techniques have resulted in heterogeneous surfaces. U.S. Pat. No. 4,536,420 to Rickert discloses a water-wettable coating made from a mixture of colloidal acrylic resin and colloidal silica which, when cured, has a mud-cracked pattern, thus providing canals in the surface which tend to break up water droplets. Japanese Kokai Patent Publication 59-176,329 to Mitsubishi Monsanto Kasei Vinyl K.K. discloses transparent molded materials having patterned surfaces of hydrophilic and hydrophobic areas. In the examples, a patterned hydrophobic material is printed onto a hydrophilic film.